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I am building a LED array and am unsure of if I have been given bad information. I was told i can use a PC PSU to power my LED array safely using resistors and rheostats to dim them. After doing some research i have found out that LEDs need constant current more than constant voltage - a PC PSU is constant voltage.

The LEDs I purchased are all 10 watts each. I will be running 11 of them and would like to use one power supply if possible. The specs on the different LEDs and hardware are below:

4 x White Foreward voltage: 9-11v Foreward current: 900 - 1050 mA

4 x Blue Foreward voltage: 9-11v Foreward current: 900 - 1050 mA

1 x Violet Foreward voltage: 9.5-10v Foreward current: 900 - 1050 mA

1 x Green Foreward voltage: 9-12v Foreward current: 900 - 1000 mA

1 x Red Foreward voltage: 6-8v Foreward current: 900 mA

11 x 2 Ohm Resistor

4 x 1 Ohm rheostat

My plan was to power all of the LEDs on the 12v line, use a resistor for each LED and a rheostat for each color wired in series (blue and white use 1 rheostat). Red would use the 5v line and a resistor and rheostat.

My question is can i use a PC PSU? Is it a bad idea considering its not constant current? Would this setup work or would it burn up the LEDs? Would there be a better way to wire or drive this? If so what kind of a driver would i need?

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"Can I use a PC PSU?" Yes, make sure there is a load and you are connecting the signal lines properly (this information is readily available on the internet)

"Is it a bad idea considering its not constant current?" Yes

"Would this setup work or would it burn up the LEDs?" It would work fine as long as you don't exceed the ratings of the LEDs

"Would there be a better way to wire or drive this?" Yes

"If so what kind of a driver would i need?" A constant current LED driver: https://www.google.com/search?q=constant+current+LED+driver#q=constant+current+LED+driver&tbm=shop (assuming you are not willing/able to build a circuit yourself)

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  • \$\begingroup\$ I am not saying your setup won't work -- it will but there is a 'better' way. Take it slow and start small. You can always start with higher resistance. Make sure the resistors can handle these power levels (I^2 * R)!!!!!! \$\endgroup\$ – HL-SDK Sep 19 '13 at 19:47
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How are you planning to wire them?

schematic

simulate this circuit – Schematic created using CircuitLab

This has a few problems. First, considering most of the leds are 9.5v minimum/typical at 1 Amp, that means you have 2.5v to drop between the resistors and the pot. Each resistor will need to dissipate up to 2.5 Watts. The pot on the blue or white sections will need to dissipate up to 16 Watts. You must size your resistors accordingly.

Secondly, 2Ω Resistor + 1Ω Pot, at 2.5v drop, that's between 833mA and 1.25A through each led, depending on the position of the pot. That's 125% of the listed current rating. That could kill them very quickly. You might consider a bigger resistor, 2.5Ω or so. That would give between 714mA and 1A of current.

Finally, the Red Led states 6v minimum/typical. You want to power it from 5v, through both a 2Ω resistor and 1Ω Pot. It would most likely not light up brightly or at all. You do not have enough voltage to cover it's forward voltage drop.

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  • \$\begingroup\$ With all these problems I don't think I'll use the PC PSU. How difficult is it to build a CC PSU? Also if I purchased a CC PSU I would need around 100 v which isn't the most common PSU and the ones I have found are very expensive. Do you have any economical suggestions on how I should proceed with this project? \$\endgroup\$ – woody Sep 23 '13 at 19:47

protected by Kortuk Sep 20 '13 at 13:33

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